This invention relates generally to aircraft structures and, more particularly, to a crashworthy composite aircraft structure.
The purpose of a crashworthy aircraft structure is to ensure the structural integrity of critical portions of the aircraft structure during a crash to prevent or minimize injury to the aircraft's occupants. To accomplish that goal, one or more crushable structures have been provided to absorb crash energy in a controlled manner.
In U.S. Pat. No. 4,593,870 (Cronkhite et al.), crushable, energy absorbing structures ("crush structures") are provided below a helicopter's fuel compartment and below its passenger compartment adjacent to the pilots' compartment. In a crash, the crush structures limit the peak load experienced by the aircraft's occupants, thereby preventing injury due to excessive decelerative forces. In addition, the helicopter's propulsion system is mounted above the passenger and fuel compartments and is supported by their structures. Should the passenger and fuel compartment structures fail and lose their structural integrity, the propulsion system could intrude into those compartments, possibly injuring the passengers and/or breaching the fuel tank, possibly causing a fire. By limiting the peak crash loads experienced by the passenger and fuel compartment structures, the crush structures act to maintain the structural integrity of those compartments.
U.S. Pat. No. 5,069,318 (Kulesha et al.) discloses a self-stabilizing crush structure which includes composite columnar stiffeners in which the cross section of each member increases from one end to the other. When such a stiffener is crushed, the crushing begins at the end having the smaller cross-sectional area and progresses toward the end having the greater cross-sectional area. While the columnar stiffeners are the chief energy absorbing members, it will be recognized that the keel beams and bulkheads of the crush structure also absorb significant energy as they are crushed.
The crush structures disclosed in Cronkhite and Kulesha comprise a complex arrangement of longitudinal keel beams, lateral bulkheads, and vertical stiffeners. Further, in both Cronkhite and Kulesha, the energy absorbing function requires structure in addition to that which reacts normal operational loads. A dual-purpose structure which functions to react normal operational loads and to absorb crash energy would simplify and lighten the aircraft's structure, be easier and cheaper to construct, and be easier to inspect.
In some aircraft the structure of the fuel compartment is sealed and the compartment serves as the aircraft's fuel tank. However, such a fuel tank is prone to developing fuel leaks. For that reason, many aircraft employ a flexible fuel cell that is contained within the fuel compartment.
A modern flexible fuel cell is constructed of a rubberized fabric and is itself crashworthy; that is, it does not require an energy absorbing structure below it to maintain structural integrity in a crash. However, if no crush structure is provided below the fuel cell, the high peak load the fuel cell experiences in a crash generates a significant hydrodynamic pressure within the fuel cell. That pressure causes significant horizontal bulging of the fuel cell.
The fuel compartment's bulkheads and sides must possess a certain inherent level of horizonal rigidity in order to react the loads associated with normal operations. If the fuel cell is in contact with those bulkheads and sides, the fuel cell bulging resulting from the peak hydrodynamic fuel pressure experienced in a crash will exceed that inherent rigidity, causing the bulkheads and sides to bulge and possibly fail. Such bulging also decreases the vertical structural rigidity of the bulkheads and sides and may result in their failure due to vertical loads. To prevent failure due to bulging, the horizontal rigidity of the fuel compartment bulkheads and sides could be increased to resist the peak pressure fuel cell bulging, which would increase the weight of the fuel cell structure. Alternately, the width and length of the fuel compartment could be increased and the fuel cell positioned horizontally within the fuel compartment so that, at peak fuel cell bulge, the load applied to the fuel compartment bulkheads and sides would not exceed their inherent horizontal rigidity. However, the latter alternative would obviously require additional fuel compartment structure, again increasing its weight. An efficient means for limiting the horizontal load on the fuel compartment's bulkheads and sides due to fuel cell bulging would provide the maximum fuel capacity for a given fuel compartment size, thereby minimizing the weight of the fuel compartment.